update credentials.h example file

src/credentials.h.example

@@ -5,4 +5,9 @@ namespace credentials {
     const String psk = "my PSK";
 
     const String mdnsHostname = "led-marquee-sign";
+
+    const String mqtt_hostname = "mqtt-broker";
+    const uint16_t mqtt_port = 1883;
+    const String mqtt_username = "user";
+    const String mqtt_password = "pass";
 }